Union-ifying RangeTblEntry

Hi all

I'm about to have to add _another_ flag to RangeTblEntry, to track
row-security expansion.

In the process I noticed the comment:
   /*    * XXX the fields applicable to only some rte kinds should be    * merged into a union.  I didn't do this yet
becausethe diffs    * would impact a lot of code that is being actively worked on.    * FIXME someday.    */
 

and it struck me that the end of the 9.4 commitfest might be a
reasonable time to do this now that PstgreSQL is subject to "pulsed"
development with commitfests.

As part of that, a number of the flag fields on RangeTblEntry into a
bitfield.

Comments?

-- Craig Ringer                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services